packagin: ship script and tool in arch-agnostic /usr/libexec

packagin: ship script and tool in arch-agnostic /usr/libexec

no need to use the architecture triple, not like we plan to support
co-installable multi-arch anyway.

Rather just use the arch-agnostic /usr/libexec dir, which the FHS
specifies for:
> Directory contains binaries for internal use only and they are not
> meant to be executed directly by users shell or scripts.

Which pretty much is what this tools are made for, in our use case
anyway.

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>